1. Field of the Invention
The present invention relates to a lens. More particularly, the present invention relates to a fixed-focus lens.
2. Description of Related Art
Referring to FIG. 1, U.S. Pat. No. 6,989,946 has disclosed a conventional fixed-focus lens 100 applied to a rear projection television (RPTV), which includes a first lens group 110 and a second lens group 120 arranged from an object side to an image side in sequence. A liquid crystal light valve 50 is placed at the image side. The first lens group 110 includes two lenses 112 and 114 arranged from the object side to the liquid crystal light valve 50 in sequence. The second lens group 120 includes seven lenses 122a, 122b, 124, 126a, 126b, 126c, and 128 arranged from the object side to the liquid crystal light valve 50 in sequence. The lenses 122a and 122b form a cemented lens 122, and the lenses 126a, 126b, and 126c form another cemented lens 126.
Since a field of view (FOV) of the conventional fixed-focus lens 100 is not large enough (the maximum FOV is about 95°), a optical path length between the lens 112 and a screen (not shown) disposed at the object side must be long enough to project an image with an appropriate size onto the screen, which results in that the volume of the RPTV is excessively large. However, if the above optical path length is reduced by increasing the FOV through adjusting the refractive power of each lens of the fixed-focus lens 100, the image projected on the screen has a great aberration.